The subject invention pertains in general to cyclone furnaces for burning ash-containing fuels, and in particular to studs used to protect the water-cooled tubes forming the slag tap outlet.
Cyclone furnaces were developed by The Babcock & Wilcox Company (B&W) in the 1940's. These cyclone furnaces have the ability to burn high-ash low-fusion temperature coals, which are particularly troublesome in pulverized coal boilers. FIG. 1 shows a cyclone furnace assembly 100, which comprises a generally horizontal barrel, typically 6 to 10 feet in diameter, attached to the side of a boiler furnace. The cyclone barrel is made up of water-cooled tubes 24, arranged in tangent-tube construction. Crushed coal is introduced through crushed coal inlet 32. Crushed coal and some air (primary 41 and tertiary 43) enter the cyclone through one or more specially designed burners on the front of the cyclone, such as radial burner 30.
In the main cyclone barrel, a swirling motion is created by the tangential addition of secondary air in the upper cyclone barrel wall through secondary air velocity dampers 45. A unique combustion pattern and circulating gas flow structure result. The products of combustion eventually leave the cyclone furnace through a re-entrant throat 60, which includes water cooled tubes 22 adapted to form a slag tap opening or outlet 20. A molten slag layer develops and advantageously coats the inside surface of the cyclone barrel. The slag drains to the bottom of the cyclone and is discharged through slag tap 20.
Cyclone furnaces are an integral part of the boiler heat absorbing circuitry and allow for a smaller boiler since about 70-90% of the original fuel ash is captured in the slag tapped out of the furnace.
For additional details of the design and operation of cyclone furnaces, the reader is referred to U.S. Pat. Nos. 2,357,301, and 5,878,700, assigned to the assignee of the present invention, and to Chapter 15 of Steam/Its Generation and Use, 41st Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., © 2005, the texts of which are hereby incorporated by reference as though fully set forth herein.
Erosion and corrosion within the Cyclone are two critical issues which require routine maintenance measures. As shown in FIG. 1, a protective wear liner, made up of replaceable wear blocks or liners 51, is used to prevent excessive erosion at coal inlet 32. The material used for these blocks is normally comprised of metal, ceramic or a combination of the two. Knuckle studs 52 are placed next to wear blocks 51.
The cyclone's wet slagging environment produces a potentially corrosive iron sulfide attack on the pressure part tubing. Referring now to FIG. 2, in areas coated by the molten slag 72, water-cooled tubes 22, 24 are typically protected by a refractory layer 74 held in place by cylindrical pin studs 53. The pin studs 53 are welded to the outside surface of the tubes 22 in a very dense pattern. For example, the “super dense” pin studding offered by The Babcock & Wilcox Company may include ¾ inch long studs with 360 or more studs per square foot.
In addition to retaining the refractory, the pin studs cool the refractory surface in contact with the corrosive slag and help retard the corrosive action. The pin studs hold the refractory in place, thereby improving the refractory life span, and the refractory in turn helps protect the pin studs. This insulation maintains the cyclone at a high enough temperature to permit adequate slag tapping from the bottom of the unit, and significantly reduces erosion and corrosion potential.
To further reduce maintenance, The Babcock & Wilcox Company developed a flat, staggered stud design 54, shown in FIG. 1, which includes a hand applied fillet weld. The flat staggered stud design 54 offered the following advantages: 1) more precise stud manufacturing and closer spacing, 2) minimum potential for channeling and accelerated wear between the studs, 3) excellent heat transfer which reduces metal temperature and erosion rates, and 4) thicker stud sizes to extend life.
To reduce erosion and corrosion in the slag tap region of a cyclone furnace, The Babcock & Wilcox Company developed a contoured flat stud, designed for the tubes that make up the cyclone slag tap. The contoured design was developed to better fit a flat stud into the slag tap region. This original contoured stud was made of B&W 800 material, and had a generally arcuate shape, being designed as an annular or ring-like segment. Referring now to FIG. 3A, FIG. 3A is a cross-sectional view of two original contoured studs 1 applied to a water-cooled tube 22. Each original contoured stud 1 had an inner circumferential edge 2 that was contoured to contact the associated tube 22. Original contoured stud 1 had an upper side 6 and a lower side 8 each of which were flat and parallel to one another. Outer circumferential edge 4 connected upper side 6 to lower side 8 at right angles and ran parallel to inner edge 2. Outer circumferential edge 4 was exposed to slag and flue gas flowing through the slag tap opening when in use.
Original contoured stud 1 had a projection 9 extending from upper side 6 and lower side 8 and terminating in inner circumferential edge 2. Projection 9 had a concave weld recess 10 (shown as concave up in FIG. 3A) connecting upper side 6 to inner edge 2. Weld recess 10 was adapted to receive a weld 12, and had a weld recess depth 11 (FIG. 4A). Projection 9 also had a concave recess 14 (shown as concave down in FIG. 3A) located opposite recess 10 and connecting lower side 8 to inner edge 2. Concave recess 14 had a recess depth 15. Recess 14 of stud 1 was symmetric with recess 10, with weld recess depth 11 and recess depth 15 being equal.
In use, stud 1 was attached to tube 22 by fillet weld 12. Weld 12 contacted water-cooled tube 22 at weld contact area 13. Stud 1 itself contacted tube 22 at a stud contact area along inner edge 2. As shown in FIG. 3A, weld contact area 13 and the stud contact area of inner edge 2 were approximately equal.
As shown in FIG. 5A, original contoured studs 1 were arranged along the water-cooled tubes using an original contoured stud arrangement 90 in which a plurality of original contoured studs 1 were disposed about water cooled tubes 22 in a vertically staggered arrangement, i.e. staggered as the studs 1 were attached vertically, not offset in the direction of slag and flue gas flow 55.
While initial trials of original contoured stud 1 and original stud arrangement 90 showed some improvement, further reductions of erosion and corrosion rates were still desirable.